Generally, surgery is performed in a sterile operating field. A sterile operating field is an area of the body that has been cleansed, with a surgical detergent and/or an antiseptic, to reduce or eliminate any surface bacteria that may be found on the body surface in and around the area of the planned surgical incision. Since this cleansing cannot completely eliminate all surface bacteria, the operating field is then covered with a sterile cover through which the incision is made, thus extending the sterile operating field up to the area of incision. The sterile cover, also referred to as a sterile drape, prevents any contact between the sterilized surgical instruments and the surface of the body. By preventing such contact, the sterile cover reduces the possibility that any remaining surface bacteria will attach to a sterile surgical instrument and be transported into the area of incision.
The unique structure of the eye, however, has made it generally difficult for surgeons to establish and maintain a sterile operating field up to the area of incision near the cornea, which has a general diameter between 11–12 mm in normal patient populations. The difficulty exists because the surgeon routinely makes an incision near or through the conjunctiva, the moist and transparent membrane that covers exposed surfaces of the eye and the eyelids, and the conjunctiva generally contains high levels of surface bacteria. Thus, the surface of the eye is the only portion of the body surface that does not allow the intense application of surgical detergents or antiseptics, due to its delicate nature, nor the application of an adherent sterile drape, as is used on the skin, due to its moist character.
In cross section (FIG. 1), the anatomy of the eye contains a surface membrane, the conjunctiva, which extends from the inner eyelid margin to the edge of the cornea 10. Where the conjunctiva covers the inner surface of the eyelid, it is referred to as the palpebral conjunctiva 20. Where the conjunctiva covers the white wall of the eye (the sclera), it is referred to as the bulbar conjunctiva 30. The space between the upper eyelid 40 and the bulbar conjunctiva 30 is referred to as the upper fornix 50 and the space between the lower eyelid 60 and the bulbar conjunctiva 30 is referred to as the lower fornix 70. The region of the eye where the bulbar conjunctiva 30 terminates into the cornea 10 is referred to as the limbus 80. The light sensitive film lining the back two-thirds of the eye is the retina 90.
Currently, the ocular surgeon uses a translucent sterile drape 100 that covers the operating field extending only up to the margins of the lower eyelid 110 and the upper eyelid 120, where it is folded over the eyelid margins, into the fornices, by the speculum 130 that also holds open the eyelids (FIG. 2). M. G. Speaker & J. A. Menikoff, Int. Ophthalmology Clinic, 33: 51–70 (1993). This form of draping, however, does not establish a sterile field beyond the margin of the eyelids 110, 120 and the margins of both the lateral canthus 140 and medial canthus 150. During ocular surgery, the surgeon typically makes an eyewall incision through the cornea 10 or the bulbar conjunctiva surface 30, well beyond the margin of the sterile drape 100, near the limbus 80 (FIG. 2). Consequently, there is a continued concern that a patient undergoing eye surgery, such as cataract extraction, may be at an increased risk to develop a post operative infection that may develop from the introduction of conjunctival surface bacteria into the site of the incision.
In eye surgery, post-operative infection of the interior of the eye is called endophthalmitis. Despite current preventative efforts, endophthalmitis occurs in approximately 1 in 1000 ocular surgical procedures and is the most feared of all postoperative complications. J. C. Javitt, S. Vitale, J. K. Canner, et al., Arch Ophthalmology, 109:1085–1089 (1991). The bacteria that cause this complication closely coincide with the typical bacteria that have been proven to reside on the conjunctival surface of the eye. In fact, genetic studies have frequently shown that the infecting organism in endophthalmitis is identical with the bacteria cultured from the conjunctival surface of the same eye. M. G. Speaker, F. A. Milch, M. K. Shah, et al., Ophthalmology, 98: 639–650 (1991).
During a typical ocular surgical procedure, surgical instruments, irrigation fluids, and intraocular implant lenses commonly contact the exposed bulbar conjunctiva. This contact often results in the transportation of conjunctival surface bacteria into the interior of the eye, F. M. Wilson, Int. Ophthalmology Clinic, 27: 97–108 (1987), sometimes resulting in a case of endophthalmitis. Surgical statistics indicate that between 26–43% of cataract extraction patients develop culture positive aqueous fluid that results from surface conjunctival bacteria that have been transported into the eye during surgery. M. G. Speaker & J. A. Menikoff, Int. Ophthalmology Clinic, 33: 51–70 (1993).
When a patient suffers from endophthalmitis, the affected eye is temporarily blinded, and there is great concern that the infection will permanently destroy or impair the patient's vision, so it is generally considered an emergency. As such, the patient may be treated with intraocular antibiotic therapy and surgical debridement. This course of therapy, however, does not always restore the patient's vision. In fact, one half of the eyes that are afflicted with endophthalmitis remain legally blind after surgery, and one out of ten is removed, shrinks, or becomes cosmetically objectionable. A. J. Kanellopoulos & F. B. Dreyer, Int. Ophthalmology Clinic, 36 (2): 97–108 (1996).
Extensive efforts have been made over the last 30 years to reduce the amount of bacteria on the surface of the conjunctiva by optimally treating the exposed conjunctiva topically with preoperative and postoperative antibiotic eye drops, and intraoperatively with subconjunctival antibiotic injection. D. C. Classen, R. S. Evans, S. L. Pestontnik, et al., New England Journal of Medicine, 326: 281–286, (1992); M. B. Starr & J. M. Lally, Survey of Ophthalmology, 39 (6): 485–501 (1995). These approaches, however, have not been completely effective in the prevention of post surgical infections. Antibiotic eye drops are cumbersome to apply in a sufficient amount to reduce conjunctival bacterial colony counts to acceptable levels. Additionally, the use of antibiotic eye drops is expensive; requires patient compliance; and exposes the patient to potential complications associated with adverse or toxic reactions. Furthermore, there is a concern that the overuse of antibiotics in prophylaxis contributes to the emergence of resistant bacterial strains that are difficult to eradicate. Finally, sub-conjunctival injections have been reported to accidentally pierce the eye wall itself, instantly destroying all central vision by chemically injuring the retina (see FIG. 1). P. A. Campochiaro & B. P. Conway, Arch Ophthalmology, 109: 946–950, (1991).
Because the concern over the development of endophthalmitis is great, and the antibiotic approaches have not been completely successful, ocular surgeons have attempted to additionally reduce conjunctival surface bacteria by using antiseptics. Unfortunately, the use of antiseptics has been limited because the ocular surface is delicate. Thus, the use of antiseptic preparation must be limited both in strength and duration to avoid toxicity. Of the potential antiseptic agents, Povidine Iodine 5% has been shown to be well tolerated and to have an additional effect to topical antibiotics in endophthalmitis prevention. M. G. Speaker & J. A. Menikoff, Ophthalmoloay, 98: 1769–1775 (1991). This effectiveness, however, is proportional to the amount of time the solution stays in contact with the surface of the conjunctiva. Thus, the use of Povidine Iodine 5% has not been completely effective, partly because it is not left on the surface of the eye for prolonged periods due to toxicity.
In addition to the potential risks associated with endophthalmitis caused by surface bacteria, complications associated with phototoxicity can also result from an ocular surgical procedure. When ocular surgery is performed, the surgeon must use a surgical microscope that requires high-powered illumination. This illumination, in the form of intense light, can damage the retina. Such damage to the retinal tissues is referred to as phototoxicity or photic retinopathy.
Although such advanced technology has vastly improved the precision of ocular surgery, the use of modern microscopes in eye surgery can paradoxically damage the retina when used without precautions. For optimal surgical viewing, the surgical microscope emits light to illuminate the surgical field. Some of this light is reflected off the conjunctiva and the sclera, and the rest is transmitted through the clear cornea to the retina. During eye surgery, the patient's natural defenses against this light are disabled. Consequently, the patient is especially vulnerable to phototoxicity of the retina.
The damaging effect of medical instrument illumination has been recognized for three decades. In 1973, Tso described photic retinopathy lesions that were intentionally produced in the eyes of a monkey. M. Tso, Investigative Ophthalmology 12: 17–34 (1973). Later, McDonald and Irvine described a group of patients after cataract extraction that had similar lesions. H. R. McDonald & A. R. Irvine, Ophthalmology 90: 945–951 (1983). Such lesions have also been intentionally demonstrated by experiments that exposed human eyes, prior to the removal of the eye for an unrelated malignant tumor, to an operating microscope light for sixty minutes. W. R. Green W R & D. M. Robertson, American Journal of Ophthalmology, 112: 520–527 (1991).
The prevalence rate of retinal phototoxicity has been estimated to range from 3% to 7.4% after surgery for cataract extraction using the illuminated microscope. S. G. Khwarg, F. A. Linstone, S. A. Daniels, et al., American Journal of Ophthalmology, 103: 255–263 (1987); J. E. Gomolin & R. K. Koehekoop, Canadian Journal of Ophthalmology, 28: 221–224 (1993). Even if a characteristic retinal burn is not present, it has been postulated that subtle, chronic cystoid edema may be caused at the visual focal point of the macula. At its worst, phototoxicity can produce permanent legal blindness in the effected eye.
Protection of the human retina against phototoxicity is partly provided by the ocular media, which filters or absorbs the most damaging ultra-violet rays. The pupil is also capable of constricting in response to bright light, thus reducing light transmission to the retina by more than 80%. R. E. Records & J. L. Brown, Adaptation in Duane's Foundations of Clinical Ophthalmoloay, vol. 2, ch. 16, Tasmas and Jaeger Editors (1991). Finally, if light is too intense, the eyelids will close, or the brain may turn the gaze of the eyes away from the offending light source.
During eye surgery, almost all of these retinal protective mechanisms may be disabled. The pupil is usually paralyzed pharmacologically in the dilated state. The human lens is often removed, allowing greater transmission of potentially damaging light to the retina. An eyelid retractor (speculum) is employed to prevent eyelid closure. Under these circumstances, a typical operating microscope on high illumination can produce a visible photic retinopathy lesion in 4 to 7.5 minutes. A. R. Irvine, I. Wood, & B. W. Morris, Trans-American Ophthalmological Society, 82: 239–260 (1984).
For over twenty years, various efforts have been made to understand and prevent phototoxicity during eye surgery. W. A. Solley & P. Sternberg, Int. Ophthalmology Clinic, 39 (2): 1–12 (1999). The filtering of shorter wavelength light lessens but does not eliminate retinal damage. R. H. Keates & P. R. Armstrong, Ophthalmic Surgery, 16: 40–41 (1985). Similarly, the filtering of infrared light greater than 700 nanometers is only partially helpful. M. A. Mainster, W. T. Ham, F. C. Dehori, Ophthalmology. 90: 927–932 (1983). The avoidance of intense illumination has been recommended as has tilting the microscope to avoid the macula. Defocusing the beam on the retina, by placement of an air bubble in the anterior chamber, does not prevent phototoxicity. The use of corneal covers and eclipse filters has been advocated, but these devices obscure the operating field and can be used only during pauses in surgery. Finally, even proximity devices have been advocated which will turn the microscope light off when the surgeon's head leaves the microscope during pauses in eye surgery. E. Urinkowsky, M. Cahane, I. Ashkenanazi, M. Blumenthal, & I. Avni, Ophthalmic Surgery, 25 (2): 122–5 (1994).